Epoxy resins form cured objects that are generally excellent in the mechanical properties, water resistance, chemical resistance, heat resistance, electrical properties and the like, when cured with various curing agents, and are used in a wide range of applications such as adhesives, coating materials, laminate plates, molding materials and casting materials.
As for the epoxy resins, those which are in a liquid state at normal temperature, or those having a softening point of about 50 to 100° C., are generally used. In recent years, the epoxy resins or cured objects thereof used in the above-described applications are required to have higher purity as well as further improvements in various properties such as heat resistance, moisture resistance, adhesiveness, low dielectric constant, fast curability, fire retardancy and high toughness. Among them, further technical development in the fields such as electric/electronic industrial applications, automobile applications and aerospace applications, has resulted in even stronger demand for superior heat resistance, moisture resistance and toughness.
Furthermore, as another problem in using epoxy resins, there may be mentioned the storage stability of the resins. That is, the methods for using an epoxy resin include a two-liquid type method where the epoxy resin is stored separately from a curing agent or the like, and the resin is mixed with the other components at the time of use, and a one-liquid type method where the epoxy resin is stored in a state mixed with a curing agent and the like from the beginning. Although the one-liquid type method is advantageous in terms of workability, there is raised a problem that the epoxy resin and the curing agent slowly react with each other during the storage, and there occur changes in the viscosity in the case of liquid compositions, or changes in the fluidity in the case of solid compositions.
Furthermore, recently photosensitive resin compositions are increasingly used because of their convenience in the curing conditions and workability. However, since simply curing with light results in low moisture resistance and heat resistance of the resin compositions, it is not possible to achieve the high degree of reliability that is required from electric/electronic materials. Thus, in recent years, photo-and-heat-curable resins in particular are attracting interest. For example, in the applications of solder resist, supplementary ink, overcoat, various adhesives and the like, there have been used epoxy resin compositions which are characterized in that an epoxy resin is added to the components of such material, and the mixture is subjected to primary curing with light, and then to secondary curing by heating. In such applications, maintaining the storage stability of epoxy resins up to the secondary curing becomes critical. For such reasons, crystalline epoxy resins are attracting attention.
As such an epoxy resin, there has been reported a crystalline tetrafunctional epoxy resin, for example, an epoxy resin obtained by glycidylating 1,1,2,2-tetrakis(hydroxyphenyl)ethane (Patent Document 1). This epoxy resin has a melting point close to 180° C., and it is reported that an epoxy resin composition containing this epoxy resin undergoes almost no change over time even if left to stand at 80° C. for a long time, and has excellent storage stability, thus resulting in high heat resistance of a cured object obtained therefrom (Patent Document 2). However, since the subject compound is highly pure, and has low compatibility with other components (for example, curing agent) in the case of curing the epoxy resin composition, it is difficult to make the crystals completely compatibilized before the initiation of curing and the formation of a uniformly cured object is difficult, and thus there remain problems in, for example, impact resistance and moisture resistance. Also, as a method for producing an epoxy resin by glycidylating 1,1,2,2-tetrakis(hydroxyphenyl) ethane, Patent Document 2 describes a method of allowing 1,1,2,2-tetrakis (hydroxyphenyl)ethane and epihalohydrin to react in a glycidylation reaction, subsequently adding water to the system, and eliminating epihalohydrin by azeotropically boiling the system, to thus precipitate the crystals of the desired epoxy resin in water. However, in this method, there are cases where the epoxy resin partially undergoes ring-opening, and impurities derived from water or epihalohydrin may remain behind in the resin. Thus, there is a problem that the total amount of chlorine remaining in the resin may become large.
Patent Document 3 also describes a method for producing such epoxy resin, the method including precipitating crystals in a convenient manner by using a solvent whose boiling point is higher by 30° C. or more than that of epihalohydrin in the glycidylation reaction. However, this method may have adverse effects since the high boiling point solvent is incorporated into the crystal system, and remains behind within the crystals even after drying the crystals. Further, Patent Document 4 uses a technique of allowing 1,1,2,2-tetrakis(hydroxyphenyl)ethane and epihalohydrin to react in a glycidylation reaction, subsequently heating and distilling off the organic solvent from the reaction liquid under reduced pressure, combining an arbitrary organic solvent with the resulting residue, and precipitating out crystals. However, since the melting point of the product is very high, there is a risk that crystallization takes place inside the reactor during the time of distilling off the solvent. Thus, this method is industrially disadvantageous, and also, the yield is poor.
Patent Document 1: Japanese Patent Application Laid-open No. 9-3162
Patent Document 2: Japanese Patent Application Laid-open No. 2004-43533
Patent Document 3: Japanese Patent Application Laid-open No. 2005-200527
Patent Document 4: Japanese Patent Application Laid-open No. 2005-220300